You can approach this in 2 ways.
1) Siphon out as much as possible while doing a water change, do a black out of the tank for 3-5 days (turn of lights and cover the tank with something so no light gets in) add a phosphate remover like ROWAphos, on the last day do another water change. Also no feeding while doing this. Don't worry your fish will be fine.
Or
2) use red slime remover (it's a medication) follow the instructions on the box. Personally i tried this approach and it didn't really work. But it might for you!